Microsoft have been getting into user-created gaming in a big way of late. First Xbox LIVE Community Games, then Kodu. Which means they have little use for their original games creator, Popfly.
Released to minimal fanfare last year as a set of templates upon which people could build games with little or no programming experience, Popfly never really caught on. Which is why, come August 24, it'll be shut down, with users asked to direct their efforts towards XNA or, for those with less time for coding, Kodu.